Optical radar for ranging and velocimetry became viable with the advent of the laser. In one method of velocity measurement, backscattered light is combined with at least a portion of the laser light to produce a beat frequency that is proportional to the velocity of the backscattering target. As taught in an article "Laser Doppler Velocimeter using the Self-Mixing Effect of a Semiconductor Laser Diode" by S. Shinohara, A. Mochizuki, H. Yoshida and Masao Sumi, Applied Optics 25, 1417 (1989), backscatter feedback into the laser directly alters the laser emission intensity. This allows for small, low power semiconductor laser diodes to be considered for compact radar systems. For example a proximity detector using these principles is disclosed in U.S. Pat. No. 4,733,609 (Goodwin et al). However, the practical range of detection with such systems has been limited to about 3 meters by the coherence length of the laser.
Target range (distance) measurement has been effected with laser diodes by modulating the DC current used to drive the laser diode, as disclosed in an article "Range Finding Using Frequency-modulated Lase Diode" by G. Behiem and K. Fritsh, Applied Optics 25, 1439 (1986). This modulation introduces a so-called chirp frequency into the laser emission. Backscatter from a target, combined with the frequency chirp, introduces an amplitude modulation of the laser output which is used for computing range. The practical operational range, however, is again limited by the coherence length.
Further results of self-coupling of a laser diode to an external reflector are presented in an article "Laser Diode Feedback Interferometer for Stabilization and Displacement Measurements", by T. Yoshino, M. Nara, S. Mnatzakanian, B. S. Lee, and T. C. Strand, Applied Optics 26, 892 (1987). This article is directed toward measurements of alterations to the optical path between the diode and the reflector, including measuring mirror displacement associated with vibration of the mirror.